What Are Greenhouse Gases and the Greenhouse Effect?

Pollution at sunrise, Castleton, Derbyshire, Peak District. UK
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Greenhouse gases trap solar heat close to Earth in the same way that insulating glass panels keep heat inside a greenhouse. The heat comes to Earth in the form of visible sunlight. Once it radiates back out from Earth it takes the form of long-wave (infrared and invisible) energy. Unimpeded, that energy would escape Earth’s atmosphere and pass into space. However, greenhouse gases absorb much of the energy, trapping it in the lower reaches of Earth’s atmosphere where it warms the planet’s oceans, waterways, and surface. The resulting temperature increase is called the greenhouse effect.

The primary greenhouse gases include carbon dioxide, methane, nitrous oxide, and a small group of synthetic chemicals called hydrofluorocarbons. Carbon dioxide is the gas most responsible for the greenhouse effect because it is the most abundant and it persists in the atmosphere for 300-1,000 years.

Greenhouse effect vector diagram
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According to the annual State of the Climate review published by the National Oceanic and Atmospheric Administration (NOAA), the 2020 atmospheric concentrations of carbon dioxide were at their highest levels ever recorded by instrumentation. They were also at higher levels than any discerned by analysis of the many minuscule particles of soot, dust, ash, salt, and bubbles that once floated in Earth’s atmosphere and have been trapped for as long as 800,000 years in glacial ice.

Not surprisingly, NASA reported that 2020 was as hot worldwide as 2016, which previously held the “hottest year ever” record. 

The Greenhouse Effect Is Anthropogenic

“Anthropogenic” means “from humans.” According to an August 2021 report from the United Nation’s Intergovernmental Panel on Climate Change (IPCC), that word describes the abundance of greenhouse gases that have been warming Earth since the Industrial Revolution. The report states, “Observed increases in well-mixed greenhouse gas (GHG) concentrations since around 1750 are unequivocally caused by human activities.”

The report also says that the modern world’s mix of anthropogenic greenhouse gases is largely generated by fossil fuel burning, agriculture, deforestation, and decomposing waste. 

Like the IPCC, the United States Environmental Protection Agency (EPA) names burning fossil fuels—most commonly for electricity, heat, and transportation—as the single largest source of greenhouse gases in the United States.

The EPA also explains that atmospheric hydrofluorocarbons (the fourth major type of greenhouse gases) are manufactured for use in refrigeration, air-conditioning, building insulation, fire extinguishing systems, and aerosols.

According to the United Nations Environment Programme, the use of hydrofluorocarbons became popular in the 1990s after an international agreement called the Montreal Protocol stipulated the phasing out of gases that deplete the ozone layer.

The Major Greenhouse Gases

  • The primary anthropogenic greenhouse gases are carbon dioxide, methane, nitrous oxide, and a small group of synthetic chemicals known as hydrofluorocarbons. 
  • The primary human sources of carbon dioxide, methane, and nitrous oxide are fossil fuel burning, agriculture, deforestation, and decomposing waste. 
  • Hydrofluorocarbons are chemicals manufactured for use in refrigeration, air-conditioning, building insulation, fire extinguishing systems, and aerosols.

Non-Anthropogenic Greenhouse Gases

A relatively small percentage of the greenhouse effect is due to naturally occurring greenhouse gases that have been produced throughout Earth’s history by normal geologic activity. In those quantities, greenhouse gases are a benefit to the planet, not a problem for it.

According to the United Nation’s World Meteorological Organization, the greenhouse effect resulting from natural geologic activity warms the average surface temperature of Earth by 33 degrees Celsius (91.4 F). Without that natural greenhouse gas effect, Earth’s average surface temperature would be about -18 degrees Celsius (-0.4 F). Earth would probably not be habitable by the lifeforms that we know today.

As beneficial as naturally generated greenhouse gases have always been, with the atmosphere in the 21st century flooded by anthropogenic greenhouse gases, the patterns of daily life on Earth are being disrupted. Islands and shorelines are flooding. Hurricanes, tornadoes, and wildfires are rampant. Coral reefs and other marine animals are dying. Polar bears are becoming stranded on broken slabs of ice. Many species of plants and animals and much of the food chain upon which animals and humans rely are imperiled.

A 2020 article published in the peer-reviewed journal Proceedings of the National Academy of Sciences of the United States of America (PNAS) presented data from 538 plant and animal species found all over the world and warned that the greenhouse effect may cause 16%-30% of those species to go extinct by 2070.

Another 2020 article, this one published in the peer-reviewed journal Nature Climate Change, predicted that, if the emission of anthropogenic greenhouse gases continues at its current pace, declining food supply along with an increase in the number of ice-free days will push polar bears into extinction by 2100.

The Current Levels of Greenhouse Gases

greenhouse gases

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Looking at atmospheric data from sampling stations around the world, in April of 2021 NOAA announced that carbon dioxide was present at 412.5 parts per million (ppm), a decrease in 2020 from the previous year of about 7%. That’s happy news, though the decrease may have been a result of the 2020 shutdown and the subsequent slowdown of economic activities including transportation.

Looking at a longer period of time, there is some very bad news in the NOAA report: since 2000, the average global concentration of carbon dioxide in the atmosphere has risen by 12%.

Methane levels surged during 2020 to 14.7 parts per billion (ppb). This is about a 6% increase over 2000 levels. Methane is far less abundant than carbon dioxide in Earth’s atmosphere, but it is 28 times as effective in trapping infrared heat reflected from Earth’s surface. What’s more, after its 10-year “lifespan,” methane oxidizes into carbon dioxide and hangs around contributing to the greenhouse effect for another 300-1,000 years.

The Greenhouse Effect and the Oceans

Oceans cover about 70%-71% of Earth’s surface. They absorb solar heat and eventually reflect it into the atmosphere, creating winds and affecting jet streams that drive weather. 

Oceans also absorb carbon dioxide from the atmosphere. According to NASA, oceans can store carbon dioxide for millions of years, keeping it out of the atmosphere altogether and stopping it from warming the planet.

As stable and successful as oceans may seem as big “carbon sinks” (places for safe sequestering of carbon), through complex biological and physiological processes, oceans respond to climate change and climate responds to oceans.

If the greenhouse effect continues to warm the world, oceanic changes will contribute to a feedback loop of unstable weather that can include both extreme heat and extreme cold. The loop could also create new regions of drought and floods that could change the face of agriculture and rural and city life everywhere. 

Meanwhile, droughts engender wildfires, which would add precipitously to atmospheric carbon dioxide loads. Carbon dioxide increases the acidity of the ocean. The resulting mineral imbalance would make it more difficult for marine animals to create the exoskeletons and shells on which many depend.

The EPA warns that changes in ocean systems typically happen over long periods of time. Whatever damage anthropogenic greenhouse gases are currently inflicting on the seas and marine life may take a very long time to overcome.

The Fix?

According to the IPCC climate report, some of the greenhouse effect may be irreversible for many generations to come. However, some changes can be slowed and maybe even stopped, but only if human-made contributions to greenhouse gas levels are slowed and stopped. 

The Paris Agreement is an international treaty adopted by the United States and 195 other nations and entities in December of 2015 and entered into force in November of 2016. It calls for bringing emissions of greenhouse gases down by the year 2050 to net zero, a value that doesn’t require the emissions to stop altogether but to be low enough to be absorbed out of the atmosphere by new and developing technologies.

The international agreement also calls for enough cooperation to bring emissions down between 2050 and 2100 to levels that can be naturally and harmlessly absorbed by soil and oceans. Scientific models suggest that these measures would limit global warming to below 2 degrees Celsius (which is 3.6 degrees Fahrenheit). 

By the terms of the Paris Agreement, each signatory to the Agreement sets its own Nationally Determined Contribution (“NDC”), a five-year set of actions and goals. There are currently only 191 parties to the Paris Agreement. The United States signed the Paris Agreement during the presidency of Barack Obama. In June of 2017, however, President Donald Trump gave notice that, effective January 20, 2020, the United States would withdraw. On February 19, 2021, less than a month after President Joe Biden’s inauguration, the United States formally rejoined the Agreement.

According to an article in the peer-reviewed journal Nature Communications, Brazil, the United States, and Japan are expected to achieve net-zero emissions earlier than the global average. China, the European Union, and Russia should achieve net-zero emissions at about an average pace, and India and Indonesia are predicted to achieve net-zero emissions later than average.

Even so, on September 17, 2021, the United Nations announced disturbing news about the Paris Agreement. The most recent 164 NDCs filed are insufficiently ambitious. Rather than trending toward net-zero, together they would allow global greenhouse gas emissions to peak in 2030 at a level 15.8% higher than the level in 2010.

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